1. Field of the Invention
The present invention relates to a light-sensitive silver halide emulsion having a high sensitivity and an improved graininess.
2. Description of the Related Art
Recently, the strictness of requirements for a photographic silver halide emulsion have been increased, and high-level requirements have been made for photographic properties, e.g., a high sensitivity, a high graininess, and a high sharpness. To satisfy these requirements, U.S. Pat. Nos. 4,434,226, 4,414,310, 4,433,048, 4,414,306, and 4,459,353 disclose techniques of using tabular grains for improving sensitivity including an improvement in color sensitization efficiency obtained by sensitizing dyes, sensitivity/graininess relationship, sharpness, and covering power. In addition, JP-A-58-113930 ("JP-A-" means Published Unexamined Japanese Patent Application), JP-A-58-113934, and JP-A-59-119350 disclose multilayered color photographic light-sensitive materials each using a tabular silver halide emulsion having an aspect ratio of 8 or more in a high-sensitivity layer and having a high sensitivity and improved graininess, sharpness, and color reproducibility.
Since, however, dyes have properties of desensitizing a silver halide emulsion, optimal spectral sensitization can be achieved in only an amount much smaller than an amount by which dyes can form a continuous monomolecular layer on the surface of each emulsion grain. Therefore, the merit of tabular grains is not effectively used under the present conditions.
It has been known for a long time that, in order to solve the above problem, a so-called internal latent image type emulsion having, inside a grain, a ripening speck (to be referred to as a "sensitivity speck" hereinafter) capable of forming a developable latent image upon exposure is effectively used. For example, U.S. Pat. No. 3,979,213 discloses the fact that the degree of desensitization of intrinsic sensitivity of an internal latent image type silver halide emulsion upon spectral sensitization is much smaller than that of a silver halide emulsion comprising grains which are equal in grain size to those of the internal latent image type silver halide emulsion and chemically sensitized only on their surfaces, and that therefore color sensitization can be effectively performed by using a large amount of color sensitizing dyes. In general, however, the sensitivity speck is said to be a very small crystal of, e.g., silver sulfide or gold sulfide epitaxially bonded to a silver halide crystal, and its existing state is unstable. Therefore, since the function as a sensitivity speck is sometimes degraded upon formation of internal latent image forming sites, the effect of improving sensitivity obtained by formation of the internal latent image forming sites is not satisfactorily achieved in conventional methods.
JP-B-44-15748 ("JP-B-" means Published Examined Japanese Patent Application) discloses a photographic silver halide emulsion sensitized by at least two types of different sensitizers, i.e., a noble metal sensitizer and an unstable selenium sensitizer.
JP-B-43-13489 discloses a photographic silver halide emulsion sensitized by at least three types of different sensitizers, i.e., a noble metal sensitizer, an unstable selenium sensitizer, and an unstable sulfur compound. However, the selenium sensitization easily produces fog, though it has a sensitizing effect superior to that of the sulfur sensitization.